This invention relates to a picture signal coding apparatus which is of particular utility when employed in the coding of still pictures.
For the reduction of the cost of picture signal transmission, there have heretofore been proposed DPCM, .DELTA.M and like coding systems, for example, in A. Habihi and G. S. Robinson "A Survey of Digital Picture Coding" Computer, 7,5, pp. 22-34 (May 1974). These known systems are called prediction coding systems, in which a prediction signal is produced based on a sample preceding a reference point and a prediction error signal which is a difference between a sample to be coded and the prediction signal is quantized for transmission. Due to the property of the picture signal, the prediction error signal occurs frequently in the range of which its amplitude is small, so that even if the prediction error signal is quantized roughly, the picture quality is not significantly deteriorated; therefore the number of bits necessary for coding can be reduced as compared with that in ordinary PCM in which a sample is coded as it is.
With these conventional prediction coding systems, however, the prediction error signal must be transmitted for every sample point, and the number of prediction error signal quantizing levels cannot be reduced significantly because of contributing factors to deterioration of the picture quality such as slope over load, granular noise, false contour, etc. For these reasons, the number of bits used is relatively large as a whole.
As a result of novelty search conducted by the European Patent Office, there was not found any particular prior literature corresponding to this invention but U.S. Pat. Nos. 3,403,226, 3,940,555 and 3,984,626 were cited as reference literatures. Now, a description will be given of U.S. Pat. No. 3,403,226 of the type in which a picture frame is divided into blocks. In this patent, one of the picture elements is selected as a typical picture element for each block and coded by the PCM technique so that it can represent, by itself, its level, and the difference between the typical picture element and each of the other picture elements of the same block is coded into a PCM code.
Accordingly, though the number of quantizing levels for the latter PCM code is smaller than that for the typical picture element, transmission of the PCM codes requires a plurality of bits, resulting in an appreciably large number of bits being needed as a whole.
An object of this invention is to provide a picture signal coding apparatus which enables coding with less bits than those used in the prior art.